I think framing this question as either "in the union" or "a rat" is pretty counterproductive. There are good reasons a person might want to learn something without pursuing it as a career.
I'd feel pretty impoverished if I was only allowed to know the skills required to do my job because learning the skills required to do someone else's job was forbidden by some cloak of secrecy by those who knew the skills.
I'm an electrical engineer and several of the testing systems we design run on various levels of industrial three phase. It would be nice to wire them up according to standard so when we hand them off to the union guys actually installing them on the factory floor we're all on the same page.
In addition, it would be nice to know the standards so that I can tell when it's done wrong. For example, I once took a shot of 480VAC to my right arm due to a system having been miswired, and I very much never want to do that again.
I seriously doubt that. The number of people willing to throw money at anything is vanishingly small.
My sense from talking to them was never that they were angry--everybody seemed to genuinely want to help me even if only to observe the strange creature. It was more that simply asking to join a class without being on an apprentice/journeyman track was so far out from left field that they had absolutely no processes that could be brought to bear.
By contrast, if you want a frosty reception, out of ignorance ask a Levantine Arabic teacher to teach you Egyptian Arabic. He didn't assault me, but I think he contemplated it.
I'd feel pretty impoverished if I was only allowed to know the skills required to do my job because learning the skills required to do someone else's job was forbidden by some cloak of secrecy by those who knew the skills.